The present invention relates to microelectronics including microelectronic wiring elements and semiconductor integrated circuits having metal interconnect structures.
Electromigration is a problem which can seriously affect the long-term reliability of microelectronic elements. The problem can be particularly serious in copper interconnects which are provided as “back end of line” (“BEOL”) structures of semiconductor integrated circuits (“ICs” or “chips”). Electromigration tends to occur at ends of horizontally oriented metal lines and locations where vertically oriented vias are joined to such metal lines, mainly because the metal lines are subject to different kinds of stresses at such locations.
Failure mechanisms include the formation of a void in the copper line and mass transport of copper which occurs along the interface of a layer of dielectric material used as a cap layer overlying the copper line. A common cause of such failure includes the positive divergence of metal ions moving downstream under force of the flow of electrons (“electron wind”). As deposited, copper lines include vacancies which are microscopic gaps between deposited particles. Over time and with application of heat and/or current, vacancies are prone to move and accumulate together to form voids having substantial size. As a result, voids tend to form in metal interconnects at locations upstream in the predominant path of electrons.